1. Field of the Invention
The present invention relates to a process for controlling the horizontal movements of yarn carrier bars, correlated with a predetermined distance between centres of the knitting needles in knitting machines, comprising the use of a plurality of stepping motors each operatively connected to a yarn carrier bar for transmitting reciprocating movements having variable-width strokes to said bar, as well as a central control unit managing working cycles carried out by said stepping motors.
By working cycle it is intended an entire plurality of movements aiming at knitting any finished pattern top be made by the knitting machine.
2. Prior Art
It is known that in fast knitting machines, such as crochet galloon looms, the formation of a manufactured article relies on the cooperation of different knitting members, such as needles, eye-pointed needles and tubular weft yarn guides or threading tubes, provided with a reciprocating movement synchronized in such a manner as to give rise to the interlacing of weft yarns engaged through the threading tubes, with the warp yarns passing through the eye-pointed needles and operatively engaging about the needles. The threading tubes are arranged in one or more rows disposed parallelly in side by side relation, each of which is supported by a corresponding yarn carrier bar through which the necessary reciprocating motions are transmitted so that the threading tubes may describe, by turns at each work stroke, a given trajectory selectively extending astride of one or more needles.
To this end, the yarn carrier bars are engaged, at the respective opposite ends, to a pair of lifting plates simultaneously driven in an oscillatory motion by a vertical-movement mechanical linkage. In addition, operating on each of the yarn carrier bars is a second mechanical linkage giving the bar itself, and therefore the corresponding threading tubes, a horizontal oscillatory movement which, in combination with the above mentioned vertical movement, makes the threading tubes describe displacements according to a curved trajectory astride of the needles. By varying each time the width of the horizontal strokes of the individual yarn carrier bars, the threading tubes are induced to selectively ride over one or more needles concurrently with the formation of each knitting stitch, so as to give origin to the desired pattern or embroidery on the manufactured article.
In knitting machines of the most usual conception, the reciprocating movement of the individual yarn carrier bars is achieved with the aid of the so-called "Glieder chains", consisting each of a plurality of suitably shaped cam elements, interlinked one after the other in an endless line. The cam elements of the individual Glieder chains, mounted on appropriate driving pulleys set in rotation, act on respective cam followers associated with the individual yarn carrier bars in order to cause the horizontal movement of the latter according to a width each time proportional to the lifting of the cam element coming into engagement with the cam follower.
The Applicant has recently developed a device that, in place of said Glieder chains, utilizes a plurality of electric stepping motors operatively connected each with one of the yarn carrier bars. The selective operation of the stepping motors is managed by a programmable electronic control box into which any programs relating to the management of the motors themselves can be easily loaded, according to a work cycle suitable to obtain the desired pattern or embroidery in the manufactured article produced by the machine. In substance, the program loaded into the electronic control box contains all information relating to the extent of the stroke to be carried out, upon command of the respective motor, by each of the yarn carrier bars, at each knitting step. In order to give the control box the possibility of stopping each stepping motor the exact moment at which the yarn carrier bar has moved by the expected amount, a plate-like element is arranged on the output shaft of each of the stepping motors, which plate-like element is provided with optical references spaced apart from each other an amount corresponding to the distance between centres of the needles. Optical detectors interlocked to the control box and combined with each of the motors detect when the optical reference passes a predetermined reading point. Therefore the control box itself is capable of evaluating the number of needles ridden over by the threading tubes as a result of the movements of each yarn carrier bar so as to stop the horizontal movement of said bar at the appropriate moment.
Each stepping motor is also equipped with a blocking mechanism adapted to intervene whenever the power supply to the knitting machine is broken, in order to ensure that the corresponding yarn carrier bar is stopped at a position adapted to enable the threading tubes to be inserted between the needles in the absence of mechanical interferences during the vertical strokes that are unavoidably carried out by the yarn carrier bars under inertia: before the knitting machine thoroughly stops. Each of these blocking mechanisms consists of a sector gear connected to the output shaft of the corresponding motor. This sector gear, the teeth of which are spaced apart an amount corresponding to the distance between centres of the needles, is designed to be engaged by a fitting wedge that, during the usual operation of the machine, is held by an electromagnet couteracting the elastic action of a spring. In the lack of current, the resulting de-energizing of the electromagnet causes the engagement of the fitting wedge between two consecutive teeth of the sector gear and, as a result, locking of the yarn carrier bar at a position adapted to avoid mechanical interferences between the threading tubes and the needles.
Although the use of stepping motors with movement devices represents an important technical progress as compared to the use of Glieder chains, said movement devices have proved to be capable of further improvements under different points of view.
For example, it has been found that detection of the optical reference passage before the reading point does not completely meet the requisite reliability and accuracy in the control of the stroke carried out by the individual yarn carrier bars. In fact, it is very difficult, above all at high operating speeds of the machine, to carry out stopping of the yarn carrier bar at a location sufficiently exact to avoid the risk of mechanical interference between the threading tubes and the needles, above all when a very high working fineness is required, that is when the distance between centres of the knitting needles is very reduced. In addition, in the case that, for any reason, one or more yarn carrier bars should undergo accidental shiftings that are not governed by the electronic control box, the control of the bar positioning would be permanently impaired as far as an operator intervenes and resets the entire movement device. This is essentially due to the fact that the electronic control box is exclusively capable of carrying out counting of the optical references passing before the reading points and does not have the possibility of executing any precise monitoring as regards the actual position of the yarn carrier bars in relation to the angular positioning of the output shafts of the stepping motors. In particular, it may happen that, due to vibrations or any other reason, an optical reference stopping at the reading point may slightly move back and, subsequently, reach again the reading point. The electronic control box would interpret such a circumstance as a displacement of the yarn carrier bar by an amount equal to the distance between centres of the needles whereas, as a matter of fact, the bar has not substantially moved.
It is also to be pointed out that in the above described device operation of the stepping motors takes place at a predetermined and constant speed that, in order to reduce the risks of mechanical interferences when the machine is running at high speeds, must correspond to the highest speed that the motors can reach. As a result the mechanical members connected to the yarn carrier bar are greatly stressed even in cases in which said bar would have to carry out a limited displacement and consequently operation of the corresponding motor could take place at a reduced speed without involving risks of mechanical interferences between the knitting members.
It will be also recognized that the plate-like elements carrying the optical references must be replaced every time the knitting machine is set up for executing workings having a fineness or stitch gauge different from the preceding one.
Also the sector gears of the above mentioned blocking mechanisms must be replaced each time the working fineness is changed and, in addition, apart from that, the presence of said sector gears makes the device as a whole much more complicated.